1. Field of the Invention
This invention relates to a polymeric electrolyte having good room temperature ionic conductivity, more particularly to a polymeric electrolyte film having good room temperature ionic conductivity and chemical stability, in which a non-volatile polar oligomer polyether or polyester with terminal groups halogenated is used as a plasticizer.
2. Description of the Prior Art
Polymeric electrolytes can be processed to form films for the production of thin film batteries with a thickness of less than 100.mu.. Such batteries have a wide variety of uses, for example as power supplies of smart cards, calculators, portable computers, electrical appliances, IC chips and solar energy storage. In addition, solid polymeric electrolytes have other uses, for example in solar batteries, monitors, sensors, and the applications in electrochemistry and photoelectrochemistry.
The main path for ion transportation in a polymeric electrolyte is via the amorphous region of a polymer matrix. Thus, the ionic conductivity of a polymeric electrolyte can be increased by diminishing the crystalline region and increasing the amorphous region of the polymer matrix. The methods frequently used are: (1) preparing a new polymer such as copolymer or polymer with network structure; (2) adding non-soluble additives to improve the electrolytic property; and (3) adding soluble additives to provide a new path for ionic conductivity.
Polymers having high-dielectric constants, including poly(vinylidene fluoride)(.epsilon.=8-13), poly(ethylene succinate)(.epsilon.=5.0-5.5), poly(ethylene adipate)(.epsilon.=5.2) and poly(acrylonitrile)(.epsilon.=3.1-4.2) are all good matrices for preparing polymeric electrolytes. However, because they all have high glass transition temperatures or a high degree of crystallinities, the resulting polymeric electrolytes are not as good as desired. Adding some polar organic solvents, such as propylene carbonate, ethylene carbonate, or N,N-dimethylformamide and butyrolactone to such polymeric electrolytes, can indeed improve the ionic conductivities thereof, however, it is still hard to attain an ionic conductivity of 10.sup.-4 S/cm at room temperature. Furthermore, the organic solvents will volatilize, and thus changes the properties of the polymeric electrolyte.
Tsuchida et al. disclose a polymeric solid electrolyte prepared by adding a poly(ethylene oxide) oligomer having an average molecular weight of 400, and a terminal hydroxy groups acetylated poly(ethylene oxide) oligomer to poly(methacrylic acid). The polymeric solid electrolyte demonstrated an ionic conductivity of about 10.sup.-6 S/cm at room temperature.[Solid State Ionics, 11, 227(1983)]
Kelly et al. disclose that the conductivity of poly(ethylene oxide) doped with lithium salt, and plasticized with poly(ethylene glycol) having terminal hydroxy groups methylated can attain about 10.sup.-5 S/cm at 30.degree. C. [J. Power Sources, 14, 13(1985)].
The conductivity of a polymeric electrolyte must be greatly improved if it is to be used in lithium-based film batteries. Moreover, since poly(ethylene glycol)s are terminated with hydroxy groups which are readily reacted with other ingredients, they are not suitable for use in polymeric electrolytes, and thus should not be used in the manufacture of thin film batteries.